1. Field of the Invention
The present invention relates to a method and apparatus for evaluating the performance of a wireless LAN (local area network) system, and more particularly, to a performance evaluation method and apparatus which can predict the throughput and the like of a wireless LAN system even before the wireless LAN system is constructed.
2. Description of the Related Art
With the proliferation of wireless LANs, a need exists for techniques for evaluating the performance, for example, the throughput and the like, of wireless LANs. The wireless LAN suffers from a lower throughput due to errors on a radio transmission, i.e., errors on a physical layer caused by the use of the space as a transmission medium, and collisions resulting from simultaneous transmissions started on the MAC (Medium Access Control) layer, attempted by a plurality of terminals. Errors on the radio transmission include, for example, those caused by radio interference, extraneous noise, and the like.
To address this problem, there is a higher need for evaluating the performance, particularly, the throughput of wireless LANs than wired LANs.
In a VoWLAN (Voice over Wireless LAN) system in which voice communications are established using a wireless LAN, packet transmission delay affects voice quality, and evaluation of the packet transmission delay is also important. The packet transmission delay is a deferment time until transmission of a packet is successfully completed when a terminal or an access point transmits the packet. The deferment time includes a channel competition time against other terminals and access points, and a re-transmission time due to a failure in transmission.
The performance of a wireless LAN may be evaluated by measuring the throughput and the like in the actual wireless LAN using a measuring instrument. However, this measurement-based approach disadvantageously encounters difficulties in providing conclusive results because this approach can be applied only to existing wireless LAN systems, and because performance values such as the throughput can largely vary depending on particular situations in which a wireless LAN is used. Taking into consideration the time and cost required to construct a wireless LAN system, the performance should be in many cases evaluated prior to the construction of the wireless LAN system rather than ex-post measurements made on the performance of the constructed wireless LAN system. For such a situation, the performance can be effectively evaluated by mathematical approaches or simulations.
For example, a performance evaluation approach based on a collision probability analysis in a wireless LAN has been proposed in G. Bianchi, “Performance Analysis of IEEE 802.11 Distributed Coordination Function,” IEEE Journal on Selected Areas in Communications, Vol. 18, No. 3, pp. 535-547, 2000. However, this approach makes an analysis premised on ideal radio channel conditions without transmission errors, thereby implying a problem that it provides evaluations too optimistic for actual systems.
Also, as described in A Doufexi, S. Armour, M. Buler, A. Nix, D. Bull, J. McGeehan, and P. Karisson, “A comparison of HIPERLAN/2 and IEEE 802.11a Wireless LAN standards,” IEEE Communications Magazine, Vol. 40, No. 5, pp. 172-180, 2002, an approach evaluates the influence exerted by transmission errors to the throughput, intended only for a single transmission/reception pair in consideration of radio conditions in the physical layer. However, this evaluation is based on a simple approach which involves subtracting an overhead as defined by the transmission standard, and a portion multiplied by an error ratio from a transmission rate as defined by the physical standard. Therefore, this approach is incapable of evaluating the influence on wireless LAN performance values such as the throughput exerted by collisions when there are a plurality of communication terminals.
Japanese Patent Laid-open Application No. 2001-168904 (JP, P2001-168904A) discloses a method of simulating the performance of a wireless LAN, which involves generating packets in accordance with a probability distribution, regarding some of generated packets as lost packets, and processing the remaining packets using a discrete event simulation to evaluate the throughput. JP, P2001-168904A, however, does not disclose how a simulation should be executed when a specific terminal topology is given.
PCT International Publication Pamphlet No. WO 00/30384 discloses a method of predicting a radio condition in a CDMA (code division multiple access) based mobile communication system when positional information on radio stations are given. This method, however, supports a prediction of errors in a physical layer, so called in the LAN, and therefore does not take into consideration collisions of transmissions on the MAC layer, so that this method, as it is, cannot be applied to evaluation on the performance of wireless LAN.
As described above, there is no method for evaluating the throughput, packet transmission delay or the like of a wireless LAN before construction of the wireless LAN. Such an evaluation method should take into consideration both transmission errors and collisions of transmissions, and could make an accurate evaluation when a specific terminal topology is given.